"I know that I haven't spent a lot of time learning the ways of Washington. But I've been there long enough to know that the ways of Washington must change," Obama said.

Obama has focused his campaign on pulling U.S. troops out of Iraq, universal health care, and stimulating the economy with a middle-class tax cut.

At a time when the majority of Americans believe the Iraq War was a mistake, Obama reminded voters that his closest Democratic rivals, Clinton and former Sen. John Edwards of North Carolina, voted to give President Bush the authority to wage the war.

A Front-Runner Falters

Last year, Clinton was considered the front-runner, with big-money donors backing her bid, not to mention a political team mirroring her husband's inner circle and an organized effort to attract women voters.

But Clinton's message of experience, Washington resume and her front-runner strategy left some Democrats cold and open to Obama's and Edwards' anti-Washington message.

Waging an outsider campaign, Obama's campaign raised a record-breaking $58 million during the first half of 2007, and boasted record-breaking support among people donating $200 and less.

Obama also attracted high-profile celebrity endorsements from talk-show queen Oprah Winfrey, and won highly sought-after Democratic establishment support from Sen. Ted Kennedy, D-Mass., Caroline Kennedy, the daughter of former President John F. Kennedy, Sen. John Kerry. D-Mass., and California first lady Maria Shriver.

As voters went to the polls in the early voting states of Iowa, New Hampshire, Nevada and South Carolina, Obama's campaign raised $36.8 million, the most ever raised in one month by a presidential candidate in the Democratic primaries.